Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geological formation, referred to as a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. Once a wellbore has been drilled, the well must be “completed”. Completion is the process in which the well is enabled to produce hydrocarbons. A completion involves the design, selection and installation of equipment and materials in or around the wellbore for conveying, pumping, or controlling the production or injection of fluids. After the well has been completed, production of oil and gas can begin.
A schematic representation of such a well 10 passing through a reservoir is shown in FIG. 1. The wellbore is typically separated from the reservoir by a perforated casing 16. Production tubing 12 is disposed concentrically within the casing 16. The production flow passes from the reservoir substantially radially into the wellbore (see arrows X), and eventually passes substantially longitudinally up the production tubing (see arrows Y).
Sand or silt flowing into a wellbore from unconsolidated formations (again, see arrows X in FIG. 1) can accumulate within the wellbore, leading to reduced production rates and damage to subsurface production equipment. Migrating sand has the possibility of packing off around the subsurface production equipment, or may enter the production tubing 12 and become carried into the production equipment. Due to its highly abrasive nature, sand contained within the production streams can result in the erosion of tubing, flowlines, valves and processing equipment. In addition to erosion, excessive sand entrained in a fluid may cause blockage of the fluid flow through the production tubing. Therefore, it is also important to measure the amount of sand entrained in a given production flow and correlate this quantity to erosion. The problems caused by sand production can significantly increase operational and maintenance expenses and can lead to a total loss of the well 10.
One means of controlling sand production is the placement of gravel (i.e. relatively large grain sand) around the exterior of a slotted, perforated, or other type liner or sand screen 14 having an outside layer usually referred to as a shroud. Amongst other things, the gravel serves as a filter to help ensure that, sand does not migrate with the produced fluids into the wellbore. In a typical gravel pack completion, the sand screen 14 is placed in the wellbore and positioned within the unconsolidated formation that is to be completed for production. The sand screen 14 is typically connected to a tool that includes a production packer and a cross-over, and the tool is in turn connected to a work or production tubing string. The gravel is mixed with a carrier fluid and pumped in slurry form down the tubing and through the cross-over, thereby flowing into the annulus between the sand screen 14 and wellbore casing 16. The carrier fluid in the slurry leaks off into the formation and/or through the sand screen 14. The sand screen 14 is designed to prevent the gravel in the slurry from flowing through it and entering into the production tubing 12. As a result, the gravel is deposited in the annulus around the sand screen 14 where it forms a gravel pack 18.
It is important to size the gravel for proper containment of the formation sand, and the sand screen 14 must be designed in a manner to prevent the flow of the gravel through the sand screen 14. However, the size of the gravel (and hence the mesh size of the screens) should not be so small as to inhibit production rates due to lower permeability. Thus gravel packs 18 and sand screens 14 can potentially permit the flow of very small particles (i.e. “fines”) through into the production tubing 12.
If fines are produced, a potential exists to cause erosion damage to the sand screen 14 and production tubing 12. The erosion damage to the sand screen 14 will depend on the erosion resistance of the sand screen 14 and the erosive properties of the produced fines under the prevailing flow conditions. If the fines begin to damage the sand screen 14 then the effectiveness of the sand screen 14 to inhibit the flow of larger sand particles is progressively diminished. As a result, potentially larger sand particles can pass through the sand screen 14. The larger mass of these particles will possess a greater capacity to cause accelerated erosion. The erosion properties of particles are strongly influenced by particle kinetic energy. The higher the particle mass and velocity, the higher is the erosion potential.
The radial flow velocity increases as the flow progresses from the formation, through the gravel pack 18 and into the sand screen 14. The radial velocity at the outlet of the sand screen 14 is at its highest and could represent the highest risk of erosion from particles flowing through the sand screen 14.
Due to the potentially complex flow regime from the reservoir into the gravel pack 18 and through the sand screen 14, as well as the potential for localised blockages, often known as “plugging”, and the potential for non-uniform sand screen material erosion resistance, the probability of a uniform erosion rate distribution throughout the sand screen 14 is unlikely. As localised erosion develops within the sand screen 14, the tendency of the flow will always be to follow the path of least resistance. This will therefore potentially further accelerate the localised erosion.
As erosion progresses, the sand screen 14 could eventually experience erosion damage of the mesh to the extent of reaching the size of the gravel. Under these conditions movement and localised flow of the gravel pack 18 could occur. This process can create gravel pack voids commencing destabilisation of the gravel pack 18 itself. This destabilisation process is often known as “fluidisation” of the gravel pack 18.
As the gravel pack 18 destabilises and fluidises, aggressive erosion conditions are created at the screen/gravel-pack interface. This highly turbulent flow regime will potentially cause further accelerated erosion of the external surface of the sand screen 14. The sand screen 14 and well 10 are now moving into the advanced stages of catastrophic failure.
Sand screens 14 and production tubing 12 are manufactured from a number of metallurgies and fabrication processes and are configured according to the specific application. Sand screens 14 and production tubing 12 are designed to optimise particle flow to minimise erosion. Each configuration will accordingly possess different levels of erosion risk dependant upon application.
The present invention seeks to provide methods and apparatuses for monitoring downhole production flow characteristics in an oil or gas well. In addition to monitoring flow conditions, it is intended that the methods and apparatuses can also provide indications of the condition of both the sand screen and the gravel pack so as to provide early warnings of potential catastrophic failure.